Recently, a so-called brushless motor (Brushless DC Motor: BLDC motor) of an electronic switching method using a semiconductor device is being widely used considering problems of mechanical contact-type of the commutator and the brush, and the brushless motor may be classified into an interior rotor type and an exterior rotor type according to arrangements of a stator and a rotor.
The interior rotor type motor uses a rotor inserting a shaft into a center of a permanent magnet in a cylindrical shape, or a so-called IPM-type permanent magnet insertion rotor inserting a shaft into a center of a rotor core, which stacks electric steel plates, and inserting a plurality of permanent magnets into a rotor core.
Recently, a permanent magnet embedded-type motor (hereinafter, IPM motor) using a reluctance torque in addition to a magnet torque as a high efficiency motor is being used. The reluctance torque is a force generated using a saliency of a d-axis inductance (Ld) and a q-axis inductance (Lq) and for this purpose, a permanent magnet has been often arranged in a V-shape.
An example of the rotor used in the permanent magnet embedded-type motor is shown in FIG. 1. Referring to FIG. 1, the rotor may be used as a driving portion for an electric compressor and the like and positioned at an inner side of a stator having a teeth protruded in an inner side direction and a coil wound around the teeth. The rotor 10 includes a rotor core 12 formed by stacking a plurality of electric steel plates. A plurality of pairs of the permanent magnets, which are arranged to form a substantially V-shape adjacent to an outer side of the rotor core 12, are fixed within the rotor core 12 in an embedded shape.
And, substantially a center of the rotor core 12 is provided with a driving shaft hole 20 inserting and fixing a driving shaft. A plurality of permanent magnet insertion holes 30 are formed at regular intervals in a V-shape which is wide open toward a stator side with respect to the driving shaft hole 20. The rotor core 12 between the driving shaft hole 20 and the permanent magnet insertion hole 30 functions as passage of a magnetic flux and also functions as supporting a rotating force of the driving shaft.
Meanwhile, there has been a problem to increase a cogging torque when increasing the number of a magnet or reducing an air gap with the stator core so as to increase the torque and efficiency in the motor as described above. If the cogging torque increases, then there are problems that noise increases and a control of the motor also becomes difficult.
Further, a plurality of permanent magnets are installed so that the driving shaft is positioned tightly close to the driving shaft hole 20 using a press fitting method and the permanent magnet closes the insertion hole with respect to the permanent magnet insertion hole 30. Thus, there is a problem that the motor may be overheated because the rotor 10 has no passage for emitting heat.
Furthermore, it is recently necessary to reduce a weight of the rotor 10 so as to achieve cost savings and enhance a rotating force.
However, in a case of forming a hole at anywhere in the rotor core 12, there may occur problems to reduce a supporting force for supporting the driving shaft and thus to make rotation unstable, and to disturb passage of the magnetic flux and thus to weaken the magnetic flux.